Despite only being ranked at #25 in the BW OU Usage Statistics in the month of March, Volcarona is undeniably one of the most dangerous sweepers in the metagame. Its ability to boost its 3 best stats by using a single move (Quiver Dance), allows it to break through many walls and rip teams apart. It has many viable sets, adding to its versatility as a sweeper and wallbreaker. Its powerful Fire-type moves can get boosted by the Sun, which dents holes even in opponents that resist its Fire moves, and make this bug even harder to counter. It does have a single downfall, though. It is 4 times weak to the common Stealth Rock. That's why it always needs Rapid Spin support to function effectively.

Moves' descriptions

Quiver Dance: Boosts the user's Special Attack, Special Defense, and Speed by 1 stage each.Fiery Dance: Deals damage and has a 50% chance to boost the user's Special Attack by one stage. The move does not give a boost if it misses or has no effect on the opponent. (80 Base Power / Fire-type)Bug Buzz: Deals damage and has a 10% chance to lower the target's Special Defense by one stage. It is a sound-based attack, so does not affect Pokemon with the ability Soundproof. (90 Base Power / Bug-type)Morning Sun: When no weather conditions are present, this move heals the user by 50% of its maximum HP. In rain, sandstorm, and hail, this move heals the user by 25% of its maximum HP. In sun, this move heals the user by 66.67% of its maximum HP.Hidden Power: Deals damage. Depending on the user's IVs, the type and Base Power of the move can be altered. (70 Base Power / Move type depending on user's IV spread)Substitute: The user loses 1/4 of its max HP and creates a Substitute. This Substitute takes damage for the user and blocks most non-damaging moves. Hurricane: Deals damage and has a 30% chance to confuse the target. In rain, this move has 100% accuracy. In sun, this move has 50% accuracy. (120 Base Power / Flying-type)Rest: The user goes to sleep for two turns, restores all HP, and is cured of burn, paralysis, and poison. Fails if the user has max HP.Movesets

For the offensive Quiver Dance set, I think that Modest is a better nature. Also, HP Ground should be slashed first if sun is being used alongside Volcarona. Just my two cents. Also, if you couldn't figure it out, Volcarona is one of my favorite Pokemon.